Friday, September 13, 2013

FPV Crash into Obstacle on RTL

This was the first time I've crashed while flying FPV. Crashes are usually caused by a series of mistakes, and this crash was no exception:

Mistake #1: Some of the hills around the flying area rose to at least 100 meters above the launch point, but I had the Return to Launch (RTL) minimum altitude set to only 50 meters above launch altitude. Note: I later realized that setting the RTL alt even higher wouldn't have helped anyway, because the current ArduCopter code foolishly limits RTL height to 80 meters).

Mistake #2: I flew behind the hill at such a low altitude that the forest cover on one of the hill's ridges came between me and the vehicle, thus scrambling my video signal (the 5.8GHz video is so high frequency that even vegetation can scramble the signal quite easily; whereas my RC control on 2.4GHz remained solid through the entire ordeal).

Mistake #3: Upon losing the video I did not back up or gain some altitude, but instead just kept going forward for a couple seconds, hoping video would come back. This just made the problem worse.

Mistake #4: When the video signal didn't come back, I switched into autopilot-controlled RTL mode, without gaining some altitude first. Thus the autopilot, already well above the set RTL minimum altitude, simply initiated a high speed beeline back towards the launch point...

RTL Code Change:

To help prevent this kind of accident happening again, I made a change to my local ArduCopter code to make it so that RTL always climbs first before heading back to launch, no matter what altitude that RTL is invoked at (although my code does provide for an absolute altitude limit if the Fence feature is turned on).

I've been using the code for a few weeks, and found that it makes FPV flying much safer, without having to set an inconveniently high RTL altitude; especially in high mountainous terrain or around tall trees. I submitted the code as the RTL Always Climb pull request to ArduCopter.


Airframe: ArduPhantom (DJI Phantom case, stock ESC, props, and battery)
Motors: T-Motor MN2214 upgrade for DJI Phantom
Autopilot: 3DR APM 2.5
Gimbal: Hummer 2-axis brushless gimbal for DJI Phantom and GoPro 3
Camera: GoPro Hero 3 Silver
GPS: 3DR ublox LEA-6H
Telemetry: 3DR 433 MHz
R/C: FlySky TH9X(ER9X FW) + 2.4GHz FrSky DJT module + 2W WiFi booster, V8R7-II rx
FPV: ImmersionRC 5.8GHz 600mA tx + CL antenna, FatShark Predator goggles + 11db patch antenna
Ground station: Mavbot Finder (first time actually used to find lost aircraft!)


  1. How did you manage to locate the quad after the crash? Did you have a visual when it went down?

    1. We used our customized ground station to find it. The ground station has it's own GPS module on it, and displays distance and direction to MAV, so we just walked in that direction until distance read zero, and there it was right at our feet.

      In case of GPS failure, our ground station also displays telemetry signal strength, so you could also walk in a random direction, note whether signal strength is increasing or decreasing, and change direction until it's increasing.